The NAICS is arranged in a hierarchical structure. The first two digits identify a major Economic Sector, the third digit designates an Economic Subsector, the fourth digit designates an Industry Group, and the fifth digit designates the NAICS Industry.
Souce: U.S Census Bureau, NAICS Code
The first two digits of the code identify the major industry group, the third digit identifies the industry group and the fourth digit identifies the industry.
Souce: NAICS Association, How to you read an SIC Code?